Add Xen to the list of virtual vendors. In the non PV (HVM) case this fixes

the virtualization detection successfully disabling the clflush instruction.
This fixes insta-panics for XEN hvm users when the hw.clflush_disable
tunable is -1 or 0 (-1 by default).

Discussed with:	jhb

sys/kern/subr_param.c

@@ -149,6 +149,7 @@ static const char *const vm_bnames[] = {
 	"QEMU",				/* QEMU */
 	"Plex86",			/* Plex86 */
 	"Bochs",			/* Bochs */
+	"Xen",				/* Xen */
 	NULL
 };